A conventional screwdriver is used for general mechanical assembling purposes such as tightening or loosening bolts. The requirement of the length of the shank of the screwdriver varies in different needs so that an extension rod is usually used so as to extend the bit to a deeper position. However, it is inconvenient for the users to carry extension rods in different lengths.
Taiwan Utility Model Publication No. M254310 discloses a retractable shank for a ratchet tool, which includes a handle with a ratchet unit received in an end of the handle and a retractable shank unit is connected with the ratchet unit. A ratchet device is connected to the ratchet unit and the shank is engaged with the ratchet device so as to rotate by the ratchet device. The shank includes an inner rod and an outer rod which accommodates the inner rod. The outer rod includes threads defined in an outer periphery of a first end thereof and is engaged with the ratchet device. The outer rod further includes two slots in a second end thereof and the inner rod has a first end thereof connected with a locking member and a second end of the inner rod is inserted into the handle. A groove is defined in an outer periphery of the second end of the inner rod. A control unit is located at the outer periphery of the shank and includes an operation member movably engaged with the two slots. A resilient member and a spring are mounted to the outer rod and a sleeve is mounted to the parts mentioned above. The operation member includes a boss at an inner periphery thereof and the boss is engaged with the groove. The operation member includes two protrusions protruding from the outer periphery thereof so as to be engaged with the slots. The resilient member and spring contact the operation member so as to control and restrict the axial movement of the shank such that the inner rod can only move axially relative to the outer rod, and cannot rotate relative to the outer rod. The inner rod connected with the locking member is retractable when using the ratchet tool. The conventional ratchet tool uses the inclination of the operation member whose inner edge contacts against the inner rod to prevent the inner rod from being retracted. The protrusions are restricted within the groove of the inner rod to prevent the inner rod from rotating during use. However, the operation member is a ring-shaped member which might be deformed when a significant torque is applied to the operation member. The protrusions can also slide off from the grooves during use and the inner rod cannot output torque to tighten bolts. The inner edge of the operation member contacts the inner rods at two points which cannot bear large torque and the operation member is therefore deformed. Besides, there are too many parts in the conventional screwdriver and may need extra maintenance.